Numerous industrial processes require accurate and precise control over the flow of different gases. Many of such processes require accurate and precise control over the quantity or mass of gas flowing into a process chamber at different time periods during the process. Examples of such processes include, without being limiting, the deposition and etching processes in which a material is deposited or etched from a substrate (such as in semi-conductor manufacture), other steps in semi-conductor manufacture, and the manufacture of medical products and devices.
Often, such gas flow control is achieved utilizing a mass flow controller (“MFC”) for each of the gases being used in the process. The quantity or mass of gas being delivered to the process over any given period of time will be a function of the gas density. Therefore, at a given volume per unit time (i.e., volumetric flow rate), there will be less mass delivered to the process per unit time for a lighter or less dense gas, for example, helium, than with a heavier or denser gas, such as nitrogen or oxygen.
A number of patents have discussed the importance of accurate, consistent delivery of gases during industrial processes and the problems associated with proper calibration of mass flow controllers (“MFCs”), and have proposed solutions to improve MFCs. Among these patents are U.S. Pat. No. 6,564,824 to Lowery et al., U.S. Pat. No. 5,791,369 to Nishino et al., U.S. Pat. No. 6,152,162 to Balazy et al., and U.S. Pat. No. 6,138,708 to Waldbusser. U.S. Pat. No. 5,744,695, to Forbes, provides an apparatus that comprises a modification of an existing gas control panel, and that can provide for calibration checks of mass flow controllers. However, the '695 patent does not utilize a bypass loop, requires all gas flow to a process chamber to be ceased in order to check one gas, and is limited in its utility in other ways. These patents, and all other patent and non-patent references cited in this disclosure, are hereby incorporated by reference into this disclosure.
Typically a gas control system for an industrial process includes an MFC for each gas, and each such MFC is uniquely calibrated to the gas flowing through that controller. Thus, each MFC provides a readout, either analog or digital, that is unique to and representative of the gas flowing through the MFC.
To maintain the desired quality of the manufacturing process, for instance a deposition or etching process used in production of microprocessor chips, the MFCs must perform within a desired range of accuracy and must provide repeatable (i.e., precise) performance. Particularly, during ongoing production operations that involve many steps to produce a high-value product, it is critical to product quality and to production efficiency to quickly identify a particular MFC that is not performing within its operational specifications.
As discussed in the above-cited patent references, achieving the ongoing accurate and precise delivery of gas through a gas-metering device such as an MFC presents a challenge. This challenge is heightened when the temperature of that gas is elevated, as this imposes additional variability upon the gas-metering device during transition from its “cold start” (when most or all parts are at room or other low temperature) to an ultimate equilibrium operating temperature (after which time temperature changes for all components of the gas-metering device are not substantial). Further, even at the equilibrium operating temperature, the performance of the device may not be reliable if the device contains certain parts that are more subject to imparting performance inaccuracy at such elevated temperature.
Also, in current industrial process systems that combine a number of gases for a single process, verification of gas flow calibration can become very difficult. Even when flow tests are conducted to determine a drift from baseline, this approach might only be able to narrow down the source of the flow rate problem to two possible gases. For example, and not meant in any way to be limiting, in the processing to deposit a Silicon Germanium film, a step requires the simultaneous flow of both GeH4 and silane or dicloro-silane (DCS). Here, GeH4 is only acting as a dopant, and a film cannot be deposited with GeH4 alone. However, GeH4 has significant impact on the deposition rate of the film. All other factors being equivalent, the greater is the flow of GeH4, the faster is the deposition rate. However, when DCS is used in combination with GeH4, an increase in DCS flow also will give an increased deposition rate. In such circumstance, if a higher than normal deposition rate is observed, the important question arises—is the GeH4 flow high or is the DCS flow high? Traditional approaches to answering questions such as this one, such as a ‘drift from baseline’ approach, are inefficient as they result in a best-guess, or trial and error effort to solve a critical production-related problem.
Thus, there is a need to provide a better approach to calibration of MFC-controlled gases in industrial processes. None of the above-cited references have provided a bypass loop approach to calibration of MFCs. The present invention advances the present state of the art by providing an apparatus, method and system to easily, repeatedly, and reliably monitor and, if needed, adjust the performance of one or more MFCs that provide one or more gases to manufacturing processes.